1. Field of the Invention
The present invention relates to a toner for visualizing latent electrostatic images by electrophotography, electrostatic recording and electrostatic printing, and a production method thereof, and a developer, toner container, process cartridge, image forming method and image forming apparatus which use the toner.
2. Description of the Related Art
Conventionally, toner is used to visualize latent electric or magnetic images in electrophotography apparatus and electrostatic recording apparatus. For example, in electrophotography, a statically charged image (latent image) is formed on a photoconductor and the latent image is developed by toner to form a toner image. The toner image is generally transferred to a recording medium such as paper and then fixed thereto by heating. The toner used for developing the statically charged image generally consists of colored particles made of colorants, charge controlling agent and other additives which are contained in a binder resin. Toner production methods are broadly classified into pulverization and suspension polymerization methods.
In the pulverization method, a colorant, a charge controlling agent, and an anti-offset agent are melt-blended and uniformly dispersed in a thermoplastic resin and the obtained composition is pulverized and classified to produce a toner. The pulverization produces toners having more or less excellent properties. However, this method sets a limit on selection of toner materials. For example, the composition obtained by melt-blending should be pulverized and classified in an economically running apparatus. For this requirement, the melt-blended composition has to be sufficiently made fragile. Thus, the particles obtained by pulverizing the composition tend to have a broad particle size distribution. In order to obtain a copied image of high resolution and high level of gray scale, fine particles having a diameter of 5 μm or smaller and coarse particles having a diameter of 20 μm or larger have to be removed by classification, which however encounters a drawback of reduced toner yield. Furthermore, in the pulverization method, it is difficult to uniformly disperse a colorant and charge controlling agent in thermoplastic resin. Non-uniform dispersion of mixed components adversely affects the toner flowability, developing property, durability, and image quality.
Recently, in order to overcome the above problems in the pulverization method, a suspension polymerization method has been proposed and practiced for toner production. Techniques for producing toners for developing latent electrostatic images in polymerization are known. For example, suspension polymerization is used to obtain toner particles. However, the toner particles obtained in suspension polymerization are spherical in shape and poor in removability. Poor toner cleaning presents no problem in the development and transfer steps when image coverage is low, because a small amount of toner particles remain after transferred. However, where image coverage is high, e.g., images such as pictures, or where toner particles remain on the photoconductor due to unsuccessful toner transfer caused by paper feed failure, the accumulated toner particles lead to background smear of the image.
Furthermore, if the charging roller for contact-charging the photoconductor is contaminated, it becomes unable to exert its intrinsic charging ability. Then, a method has been proposed in which fine resin particles are obtained by emulsion polymerization and associated to produce toner particles that are irregular in shape (see Japanese Patent (JP-B) No. 2537503). However, the toner particles obtained by emulsion polymerization have a large amount of surfactant remains not only on the surface but also inside thereof, thereby impairing the toner's charge stability against the environment, broadening the charge amount distribution, and leading to high levels of background smear. The residual surfactant further contaminates the photoconductor, charging roller, developing roller, etc., and thus these members fail to exert their intrinsic charging ability.
As for the fixing system in the electrophotography, a heating roller fixing system is widely used for its high energy efficiency and in view of device miniaturization, in which system a heating roller that is excellent in heat efficiency is directly pressed against a toner image on the recording medium for fixing. Considering the environment-friendly policy including energy-saving, lower power consumption is desired for the heating roller in the fixing step.
In attempts to resolve the above problem, fixing units have been improved and rollers have a reduced thickness on the side in contact with the toner image carrier surface for further increased heat energy efficiency, realizing a significant reduction in start-up time. However, the reduced specific heat capacity has caused difference in temperature between the area where the recording medium passes through and the area where the recording medium does not. Then, a so-called hot offset phenomenon occurs in which toner melts and adheres to the fixing roller and, after one rotation of the fixing roller, this toner is fixed to non-image areas on a recording medium. Therefore, there is an increasing demand for hot offset resistant toner.
Recently, the heat energy applied to the toner during the fixing tends to be reduced such as in the low-temperature fixing for energy-saving and high speed copy operation.
The toner used in the low-temperature fixing generally uses low-softening point resins or waxes for improved low-temperature fixing property. The low-temperature fixing toner, which is vulnerable to heat, is known to undergo so-called blocking—a phenomenon where toner particles solidify—due to heat generated from the machine and during storage. It is difficult to ensure a sufficient range of low-fixing temperature. Even with the use of polyester resin that is said to have relatively good heat storage stability in spite of its good low-temperature fixing property, no toner that resolves the above problems has been provided.
In order to satisfy the above conflicting properties, a method of producing a toner having a multilayer structure in which the particles have inner and outer resins having different compositions has been proposed. In this method, the particles have a resin having a low glass-transition temperature for improved low-temperature fixing in their interior and a resin having a high glass-transition temperature for required heat-resistance/storage stability on the surface. In this way, a toner having excellent fixing property is provided.
Proposed methods of producing a multilayer structure toner include in situ polymerization, interfacial polymerization, coacervation, spray-drying, and phase transfer emulsification. A method of producing a toner in the phase transfer emulsification has been proposed in which the toner has a multilayer structure and fine particles having a high glass-transition temperature are fixed on the toner surface for improved heat-resistance/storage stability (Japanese Patent Application Laid-Open (JP-A) No. 2001-022117). This technique allows the multilayer structure toner to have improved heat-resistance/storage stability. However, this technique does not always yield a toner with a sufficient range of fixing temperatures. Particularly, it fails to ensure the offset resistance while keeping the fixing start temperature low.
Among attempts to produce a multilayer structure toner, a toner production method has been proposed in which the particles have resins different in molecular weight between the inner and outer layers (Japanese Patent (JP-B) No. 2794770). In this method, the particles have a resin having a low molecular weight in the interior and a resin having a molecular weight higher than the interior resin on the surface, thereby providing a highly durable toner. This toner has improved durability; however, the toner does not have a sufficient range of fixing temperatures because the surface is uniformly covered with a high molecular weight substance. Particularly, it fails to ensure both the lower fixing start temperature and the offset resistance.
A toner for developing statically charged images has been proposed (Japanese Patent (JP-B) No. 3640918) that is obtained by dissolving or dispersing toner components including a toner binder consisting of a modified polyester resin reactive with active hydrogen in an organic solvent to form a solution or dispersion, reacting the solution or dispersion with a crosslinker or extension agent in an aqueous medium containing fine resin particles that may form an aqueous dispersion, removing the solvent from the obtained dispersion, and washing away the fine resin particles attached to the toner surface, wherein the residual rate of the fine resin particles remaining on the toner particle surface is 2.5% by mass or lower based on the toner particles as measured by pyrolysis gas chromatography (mass spectrometry).
By covering with fine resin particles the surface of a toner made of polyester resin having excellent low-temperature fixing property, the toner offers excellent low-temperature fixing property and heat resistance/storage stability. The toner further has a narrow particle distribution because the fine resin particles serve as a dispersion stabilizer during toner preparation. Therefore, a toner that offers excellent image quality can be obtained. However, again, such a toner does not sufficiently exert a low-temperature fixing property intrinsic to the polyester resin due to the presence of the high molecular weight fine resin particles attached to the toner surface. When the fine resin particles are used in smaller amounts, the particle size distribution of the toner may be broadened.